A wide variety of cardiac pacemakers are known and commercially available. Pacemakers are generally characterized by which chambers of the heart they are capable of sensing, the chambers to which they deliver pacing stimuli, and their responses, if any, to sensed intrinsic electrical cardiac activity. Some pacemakers deliver pacing stimuli at fixed, regular intervals without regard to naturally occurring cardiac activity. More commonly, however, pacemakers sense electrical cardiac activity in one or both of the chambers of the heart, and inhibit or trigger delivery of pacing stimuli to the heart based on the occurrence and recognition of sensed intrinsic electrical events. A so-called "VVI" pacemaker, for example, senses electrical cardiac activity in the ventricle of the patient's heart, and delivers pacing stimuli to the ventricle only in the absence of electrical signals indicative of natural ventricular contractions. A "DDD" pacemaker, on the other hand, senses electrical signals in both the atrium and ventricle of the patient's heart, and delivers trial pacing stimuli in the absence of signals indicative of natural trial contractions, and ventricular pacing stimuli in the absence of signals indicative of natural ventricular contractions. The delivery of each pacing stimulus by a DDD pacemaker is synchronized with prior sensed or paced events.
Pacemakers are also known which respond to other types of physiologically-based signals, such as signals from sensors for measuring the pressure inside the patient's ventricle or for measuring the level of the patient's physical activity.
In any pacemaker which delivers stimulating pulses in response to the presence or absence of natural electrical cardiac activity, the sensitivity of the pacemaker to natural cardiac signals is of critical importance. A pacemaker's sense amplifier circuitry must be sensitive enough to ensure detection of cardiac signals, which are typically of relatively low-magnitude (on the order of one to ten millivolts or so), especially in the case of trial sensing. A pacemaker having sense amplifier circuitry that is not sensitive enough might lose synchronization with natural cardiac rhythm, or deliver pacing stimuli at inappropriate times, if all cardiac events are not sensed. This phenomenon is known as "undersensing". At the same time, however, the pacemaker's sense amplifier should not be so sensitive that certain non-electrical cardiac signals, electromagnetic noise, myopotentials, and the like, cause the pacemaker to erroneously "sense" a cardiac event which did not actually occur. Such false sensing, also known as "oversensing", is undesirable since oversensing, like undersensing, can cause the pacemaker to deliver pacing stimuli at inappropriate times, can cause pacing at high rates, can initiate pacemaker-mediated tachycardia (PMT), can cause the pacemaker to lose synchronization with the patient's natural cardiac activity or can totally inhibit pacing.
A pacemaker's ability to sense cardiac signals is typically controllable by means of circuitry for adjusting the sensitivity threshold of the pacemaker's sense amplifier, such that electrical signals resulting from depolarization of the cardiac muscle must exceed this threshold in order for the cardiac event to be recognized. A pacemaker's sensing threshold is typically defined in terms of a minimum voltage level for the input signal, or a minimum time period during which the input signal must exceed a minimum voltage level.
In some pacemakers, the sensitivity threshold of the sense amplifier is preset during manufacture. Other pacemakers have adjustable sensitivity threshold sense amplifiers which must be set by the implanting physician prior to implant. In U.S. Pat. No. 4,640,285 to DeCote, Jr. et al., for example, an external device coupled between a pacemaker to be implanted and the pacemaker's leads is used to establish a sufficient safety margin between the pacemaker's sensing threshold and natural cardiac activity. Once the safety margin has been established, the pacemaker's threshold is programmed accordingly, and the pacemaker is implanted. In still other pacemakers, the sensitivity threshold may be adjusted after implantation by means of an external programming device. With such programmable pacemakers, the pacemaker's response to the patient's natural cardiac activity can be monitored after implantation, such as on an EKG monitor, and the sensitivity adjusted to a desired level.
One complication relating to setting the sensitivity of a pacemaker's sense amplifier is that the strength of cardiac signals received at the sense amplifier's inputs may change over time. Thus, a sensitivity threshold that is appropriate for a patient at the time of implantation might, at some later time, prove to be too high or too low, leading to undersensing or oversensing. Changes in the strength of electrical cardiac signals received by the pacemaker may result from normal or pathological changes in the heart's intrinsic activity, or from so-called "lead maturation" effects, including changes in the positioning of implanted leads, or changes in the conductive properties of the heart muscle in the region surrounding the leads, such as might result from myocardial infarction and fibrotic tissue growth around the lead. Pacemakers in which the sensitivity is adjusted only at the time of manufacture can only compensate for the occurrence of such changes by having a relatively low sensing threshold, so that even weak cardiac signals may be detected. This increases the probability of oversensing, by making the pacemaker prone to false sensing of electrical noise or myopotentials. Pacemakers in which the sensing threshold is adjusted only at the time of manufacture must be explanted and replaced or the sensing lead repositioned in order to compensate for changes in the strength of received electrical signals. Pacemakers which are programmable after implantation with an external programming device are preferable to fixed-sensitivity pacemakers, but a visit to the pacemaker clinic is required. This increases the necessity and frequency of follow-up consultations between the physician and the patient, which can be costly and inconvenient. In addition, transiently occurring changes will often go undetected.
Several attempts in the prior art have been made to cause a pacemaker's sense amplifier threshold to be automatically adjusted and maintained at an appropriate level for a given patient. Such automatic adjustment has the advantage of reducing the likelihood of oversensing, since the threshold need not be set to some low, fixed level, and also reduces the need for follow-up visits to the physician's office. In U.S. Pat. No. 4,766,902 to Schroeppel, for example, there is disclosed a pacemaker having two separate sense amplifiers. One of the sense amplifiers has a slightly lower sensing threshold, and thus a slightly higher sensitivity level, than the other. The thresholds of the two sense amplifiers are automatically adjusted so that the more sensitive sense amplifier will sense cardiac activity but the less sensitive one will not.
Similarly, in U.S. Pat. No. 4,768,511 to DeCote, there is disclosed a pacemaker having a single, fixed-gain sense amplifier. The output of the sense amplifier is coupled to a pair of voltage comparators, one of which has a slightly lower threshold than the other. The thresholds of the two voltage comparators are automatically adjusted so that one of the voltage comparators will respond to the output of the sense amplifier while the other one will not.
Another technique which has been proposed in the prior art to provide for the automatic adjustment of a pacemaker's sensitivity is disclosed in U.S. Pat. No. 4,708,144 to Hamilton et al. According to this patent, the pacemaker measures the peak value of each detected intrinsic ventricular event, and adjusts the pacemaker's sensitivity according to a long-term average of these values.
In U.S. Pat. Nos. 4,903,699 and 4,880,004 to Baker, Jr. et al. there is disclosed a pacemaker sense amplifier having automatic gain control (AGC). The sense amplifier sets a sense threshold and a slightly higher AGC threshold, such that a cardiac event which exceeds the sense threshold but not the AGC threshold indicates that the gain of the amplifier should be increased, while a cardiac event which exceeds both the sense and AGC thresholds indicates that the gain of the amplifier should be decreased.
In U.S. Pat. No. 4,827,934 to Ekwall there is disclosed a sensitivity adjustment scheme in which the amount of time during which the cardiac signal exceeds a preset threshold is used to determine whether the sensitivity of the sense amplifier should be increased or decreased.
In each of the aforementioned techniques in the prior art for providing automatic adjustment and/or maintenance of the sensitivity of a pacemaker, the determination whether to increase or decrease the sensitivity is made based on some analysis of electrical cardiac activity detected by the sense amplifier. One disadvantage of such techniques is that if no electrical activity is sensed, no determination of the appropriate sensitivity level can be made. In addition, techniques based solely on analysis of sensed electrical cardiac activity are susceptible to problems arising from extraneous electrical signals which may be present, such as myopotentials or electro-magnetic interference from external sources.
Another disadvantage of prior techniques for automatic adjustment of sensitivity thresholds in pacemakers arises from the prior techniques' failure to account for natural beat-to-beat variations in the magnitude of sensed electrical cardiac signals. That is, prior threshold adjustment schemes cannot distinguish between changes in peak sense levels that result from some form of lead maturation or chronic changes in intrinsic cardiac activity (i.e., changes which require adjustment of the sensitivity), and changes in peak sense levels that result from natural variation in intrinsic electrical cardiac activity (which do not require adjustment of the sensitivity).
It is accordingly a feature of the present invention that a scheme for automatically adjusting the sensitivity of a pacemaker sense amplifier is provided.
It is another feature of the present invention that adjustment of the pacemaker's sensitivity threshold is made based on true changes in sensed electrical cardiac signals, rather than on normal beat-to-beat variations in sensed electrical cardiac signals.
Another feature of the present invention is that effects of extraneous electrical signals such as external electromagnetic interference, myopotentials, and the like, on the adjustment of the pacemaker's sensitivity threshold, are minimized.